Geological Society of Glasgow, 145 



3. Sed.— Bed Marl. 

 Trias. \ 2. Cal. — Muschelkalk (Germany). 

 1. Sed. — New Ked Sandstone. 



!3. Sed. — Marls and Lr. Bunter Sandstone (Saxony). 

 2. Cal. — Magnesian Limestone. 

 1. Sed. — Conglomerates, Marls, and Sandstones (Rothe- 

 todte-liegende). 

 Carboni- ( 3. Sed. — Yoredale beds, Millstone Grit, Coal Measures, 

 ferous. I 2. Cal. — Limestone. 

 England. ( 1. Sed. — Lower Shales, Yellow Sandstone (Ireland). 



3. Sed. ^. /- Sandstones, / Schists, Sandstones, 



Schists, etc. . ( etc. 



Tk ^^ ' ; 2. Cal. "m ) Ilfracombe § 1 Eifel Limestone. 



JJevonian < ^ \ t ■ i. i.*a^ 



o j Limestone, etc. '^ i 



^ f a^i,,'^j.^ a J W 



1. Sed. ^ f Schists, Sand- [ Spirifer Sandstone 



^ V stones, etc. V Group. 



Tj ( 3. Sed. — Tilestones and Upper Ludlow beds. 



a,-i -^ \ 2. Cal. — Aymestry and Wenlock series. 

 Silurian. | j_ s,a.-my Hill Group. 



For the purposes of determining the directions in which the two 

 classes of sedimentary and calcareous strata augmented and decreased 

 in thickness, leading to a knowledge of the position on the globe of 

 the continents and open seas of former geologic times, Iso-metric lines 

 were proposed. These lines, drawn over the areas occupied by the 

 formations, and each line representing a certain thickness of its 

 special class (calcareous or sedimentary) of strata, would be found 

 to lead to very interesting results. Mr. Hull had already drawn 

 these lines for the Carboniferous group of Great Britain, and they 

 are shown in the map published by the Geological Society of London 

 (Quart. Journ., vol. xviii., p. 127) ; and he had no doubt that if 

 similar lines were drawn for all formations in different countries, an 

 amount of light would be thrown on the physical condition of the 

 respective groups that would considerably advance this branch of 

 science. The tracing of these lines had thrown much light on the 

 physical geology of the Carboniferous group, showing that the cal- 

 careous member (mountain limestone) attained its maximum de- 

 velopment in central England, and the sedimentary member in the 

 North, leading to the inference that an old North Atlantic continent 

 was the original source of the sedimentary materials. 



The reason of the comparative thinness of the sedimentary strata 

 of the Carboniferous series in Scotland, as compared with those in 

 North Lancashire, was then explained, on the ground of the shallow- 

 ness of the sea bed ; the impediment to the transportation of materials 

 presented by the Highlands (only partially submerged) and the in- 

 completeness of the series. It had also been found that the sedi- 

 mentary beds of the Triassic group, and of the succeeding Jurassic 

 group of England, swell out towards the north ; and the lecturer 

 expressed his belief that originally all the limestones of this series 

 tailed out or passed into sedimentary strata northwards, while the 

 clays, shales, and sandstones of the same group attained the highest 



